Note: Descriptions are shown in the official language in which they were submitted.
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PIPE CENTRALIZER HAVING LOW-FRICTION COATING
Field of the Invention
The present disclosure relates to the field of hydrocarbon recovery
operations. More
specifically, the present invention relates to pipe centralizers such as may
be used to centralize a
casing string within a wellbore.
BACKGROUND OF THE INVENTION
This section is intended to introduce various aspects of the art, which may be
associated
with exemplary embodiments of the present disclosure. This discussion is
believed to assist in
providing a framework to facilitate a better understanding of particular
aspects of the present
disclosure. Accordingly, it should be understood that this section should be
read in this light,
and not necessarily as admissions of prior art.
Technology in the Field of the Invention
In the drilling of oil and gas wells, a wellbore is formed using a drill bit
that is urged
downwardly at a lower end of a drill string. After drilling to a predetermined
depth, the drill
string and bit are removed and the wellbore is lined with a string of casing.
An annular area is
thus formed between the string of casing and the surrounding formations.
A cementing operation is typically conducted in order to fill or "squeeze" the
annular
area with cement. The combination of cement and casing strengthens the
wellbore and facilitates
the isolation of formations behind the casing.
It is common to place several strings of casing having progressively smaller
outer
diameters into the wellbore. The process of drilling and then cementing
progressively smaller
strings of casing is repeated several times until the well has reached total
depth. The final string
of casing, referred to as a production casing, is cemented in place. This is a
tubular body that
resides adjacent one or more producing reservoirs, or "pay zones." The
production casing is
frequently in the form of a liner, that is, a tubular body that is not tied to
the surface, but is hung
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from a next lowest string of casing using a liner hanger. In either instance,
the production casing
is perforated to provide fluid communication between the reservoir and the
production tubing.
In connection with setting casing strings within a wellbore, it is desirable
that the casing
strings be centered within the wellbore. In this way, the cement can flow
evenly around the
casing string, creating a more uniform barrier around the casing within the
wellbore. This, in
turn, helps to seal the annular area from fluid flow, providing sealing
integrity between
surrounding subsurface formations.
In order to center the casing string, it is known to use so-called
centralizers. Centralizers
are generally tubular bodies having an inner diameter that lightly engages the
outer diameter of a
casing string, meaning that the centralizers direct or maintain the casing
string generally
concentrically within the wellbore as the casing string is run into the hole
and when the casing is
set. Traditionally, centralizers have employed a pair of rings, or collars,
that are separated by
bow springs. The centralizers are clamped to the pipe during the run-in
process using end collars
that are hinged. In this respect, the centralizer collars are opened to mount
to the pipe, and are
then closed and secured around the pipe. Examples of such centralizers are
shown and described
in U.S. Patent No. 2,605,844 ("Casing Centralizer"); U.S. Patent No. 2,845,128
("Casing
Centralizer and Wall Scratcher"); U.S. Patent No. 2,849,071 ("Casing
Centralizers") and U.S.
Patent No. 4,531,582 ("Well Conduit Centralizer").
The process of running in a casing string with centralizers causes significant
friction to
occur between the bow springs (sometimes referred to as leaf springs) and the
surrounding rock
formation. This is in the form of drag friction. In addition, with the ever-
increasing use of
lateral and horizontal wellbores, bow springs are being asked to support a
casing string that is
being pushed laterally through a rock formation. In this respect, the casing
strings are pushed
through a deviated wellbore portion, and then in some cases across an extended
substantially
horizontal portion. The horizontal portion may extend for thousands of feet.
In order to increase the durability of the centralizer, it has been suggested
to use a solid-
body casing centralizer fabricated from minable carbon steel. TDTech, Ltd. of
New Zealand
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offers such as a centralizer, known as a SidewinderTM. The SidewinderTM tool
employs so-called
ridge-riding collars that enable a casing string to ride over ridges in the
wellbore.
To enhance the ability of joints of drill string to move through a wellbore
during drilling,
it has also been suggested to use sleeves coated with a pliable material. U.S.
Pat. No. 4,182,424
("Drill Steel Centralizer") discloses such a centralizer. Rubber or plastic
sleeves with blades that
are rigid enough to take the impacts during string delivery have also been
used as illustrated in
U.S. Pat. Nos. 4,938,299 ("Flexible Centralizer"); 5,908,072 ("Non-Metallic
Centralizer for
Casing"); 6,283,205 ("Polymeric Centralizer"); and 7,159,668 ("Centralizer").
However, this
adds complexity and expense to the manufacturing process and does nothing to
reduce friction
along surfaces contacting casing joints.
U.S. Patent Publication No. 2008/0236842, entitled "Downhole Oilfield
Apparatus
Comprising a Diamond-Like Carbon Coating and Methods of Use," discloses the
use of DLC
coatings on downhole devices. However, DLC coatings are generally cost
prohibitive for
centralizers. Recently, companies have begun offering centralizers fabricated
from plastic
materials. However, it is believed that these products do not have the
durability needed for
extended reach lateral / horizontal wellbores.
A need exists for a centralizer having a reduced coefficient of friction along
an inner
surface. This allows a casing string or a string of drill pipe to rotate and
translate between the
casing collars more freely. Further, a need exists to offer a centralizer
design having a low-
coefficient of friction coating along at least the inner surface, and
preferably also along the outer
surface. Still further, a need exists for a centralizer design having a
coefficient of friction that is
less than about 0.15.
BRIEF SUMMARY OF THE INVENTION
A centralizer for a tubular body is first provided herein. The centralizer is
designed to be
placed in a wellbore, such as a wellbore being completed for the production of
hydrocarbon
fluids.
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In one aspect, the centralizer includes an elongated body having a bore there
through.
The bore is dimensioned to receive a tubular body such as a joint of casing.
Preferably, the
centralizer defines a substantially solid body having an inner surface and an
outer surface. The
outer surface defines centralizing members such as blades disposed equi-
distantly around the
outer surface.
The centralizer also has a coating deposited on the inner surface, or a layer
formed as the
inner surface. The coating or layer is designed to provide a highly reduced
coefficient of
friction. The inner surface may comprise, for example, (i)
polytetrafluoroethylene (PTFE), (ii)
perfluoroalkoxy polymer resin (PFA), (iii) fluorinated ethylene propylene
copolymer (FEP), (iv)
ethylene chlorotrifluoroethylene (ECTFE), (v) a copolymer of ethylene and
tetrafluoroethylene
(ETFE), (vi) polyetheretherketone, (vii) carbon reinforced
polyetheretherketone, (viii)
polyphthalamide, (ix) polyvinylidene fluoride (PVDF), (x) polyphenylene
sulphide, (xi)
polyetherimide, (xii) polyethylene, or (xiii) polysulphone.
Alternatively, the coating may comprise, for example, graphite, Molybdenum
disulfide
(MoS2), hexagonal Boron Nitride (hBN), or combinations thereof.
In one aspect, the low-friction layer resides only on the inner surface of the
centralizer.
This provides for significantly reduced friction relative to a casing wall,
allowing the casing to
rotate and translate relative to the centralizer as a casing string is run
into a wellbore while still
being centralized. In another aspect, the layer also resides along blades on
the outer surface of
the centralizer. This reduces drag friction and abrasion as the casing with
attached centralizers is
run into the wellbore.
In one aspect, the coating first comprises a nitrogen-enriched coating applied
on all
surfaces. The coating is applied using a ferritic carburizing method, and
comprises a first
coating. Thereafter, a second coating of graphite, Molybdenum disulfide
(MoS2), hexagonal
Boron Nitride (hBN), polytetrafluoroethylene (PTFE) or combinations thereof is
deposited. This
may be applied using a spraying or painting process.
A method of manufacturing a centralizer is also provided herein. The method
may first
include providing a centralizer. The centralizer may have been formed, for
example, from a
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milling process. Preferably, the centralizer comprises a metal material such
as steel, though it
may alternatively comprise ceramic. As an alternative, a molding process may
be employed.
The method further involves placing the centralizer into a deposition chamber.
The
chamber preferably comprises one or more nozzles used for vapor deposition,
such as physical
vapor deposition wherein thin layers of metal are bonded onto the surfaces of
the centralizer.
Physical vapor deposition may include the disbursement of an atomized gaseous
material into the
chamber, with the atoms impregnating the centralizer surfaces at high
temperatures. Here, the
vapor is injected through one or more atomizing nozzles.
The method optionally includes heating the chamber. Preferably, the chamber is
heated
to a temperature of at least 750 F. More preferably, the temperature in the
chamber is raised to
between about 950 F and 1,150 F prior to or during the deposition process.
The processing of
heating the chamber also heats the metal material making up the centralizer.
In another aspect, the surfaces of the centralizer are heated using a plasma
torch. The
plasma torch enables heating of the downhole device to a very high
temperature, even in excess
of 2,500 F.
The method further optionally includes lowering the pressure in the chamber
during
deposition. In one aspect, the pressure is lowered to between about one and
ten tons. This
assists the deposition process.
The method also includes directing a vapor or gaseous material through the
nozzles and
onto the surfaces of the centralizer. Preferably, a gaseous mixture comprising
nitrogen and
carbon is injected through the one or more nozzles. The inert gas atoms locate
onto the
centralizer structure through a nitro-carburizing process. Further, and as a
result of the heating,
the metal material making up the centralizer expands, allowing the gaseous
mixture to penetrate
into the metal material as nano-particles.
It is preferred that the heating and vapor deposition process be conducted
over a period of
about one hour. Preferably, a ferritic nitro-carburizing process is employed
that produces a
polytetrafluoroethylene (PTFE) coating on all surfaces of the centralizer.
Thereafter, the
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deposition chamber is allowed to cool. As the centralizer cools within the
deposition chamber,
the inert nano-particles become trapped or embedded into the metal material.
In this way, a low-
friction coating is formed.
The deposition process results in a first coating being formed. In accordance
with certain
aspects of the method, a second coating is also provided over the first
coating. The second
coating consists of graphite, Molybdenum disulfide (MoS2), hexagonal Boron
Nitride (hBN), or
combinations thereof This may be applied using a spraying or painting process,
preferably at
ambient conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the present inventions can be better understood,
certain
illustrations, charts and/or flow charts are appended hereto. It is to be
noted, however, that the
drawings illustrate only selected embodiments of the inventions and are
therefore not to be
considered limiting of scope, for the inventions may admit to other equally
effective
embodiments and applications.
Figure 1A is a perspective views of a centralizer as may be used in the
present invention,
in one embodiment. The centralizer may be used for centering a tubular body
such as a joint of
casing, a liner, a joint of drill string, an injection tubing, or a sand
screen in a wellbore.
Figure 1B is a side view of the centralizer of Figure 1A.
Figure 2A is a perspective view of a casing centralizer as may be used in the
present
invention, in an alternate embodiment.
Figure 2B is a side view of the casing centralizer of Figure 2A.
Figure 3 is a perspective view of a casing centralizer as may be used in the
present
invention, in another alternate embodiment.
Figure 4 is a perspective view of a casing centralizer as may be used in the
present
invention, in still another embodiment.
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Figure 5 is a side view of a centralizer as may be used in the methods of the
present
invention, in still another embodiment.
Figure 6 is a flow chart showing steps for creating the centralizer of any of
Figures 1
through 5, in one embodiment. The method involves placing a coating of low-
friction material
onto surfaces of the centralizer.
Figure 7 is a flow chart showing steps for creating the centralizer of any of
Figures 1
through 5, in an alternate embodiment. The method involves placing the
centralizer into a
deposition chamber and conducting physical vapor deposition.
Figure 8 is a flow chart showing steps for setting a casing string in a
wellbore, in one
embodiment.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
Definitions
For purposes of the present application, it will be understood that the term
"hydrocarbon"
refers to an organic compound that includes primarily, if not exclusively, the
elements hydrogen
and carbon. Hydrocarbons may also include other elements, such as, but not
limited to,
halogens, metallic elements, nitrogen, oxygen, and/or sulfur.
As used herein, the term "hydrocarbon fluids" refers to a hydrocarbon or
mixtures of
hydrocarbons that are gases or liquids. For example, hydrocarbon fluids may
include a
hydrocarbon or mixtures of hydrocarbons that are gases or liquids at formation
conditions, at
processing conditions or at ambient conditions (15 C or 20 C and 1 atm
pressure).
Hydrocarbon fluids may include, for example, oil, natural gas, coalbed
methane, shale oil,
pyrolysis oil, pyrolysis gas, a pyrolysis product of coal, and other
hydrocarbons that are in a
gaseous or liquid state.
As used herein, the term "wellbore fluids" means water, mud, hydrocarbon
fluids,
formation fluids, or any other fluids that may be within a string of drill
pipe during a drilling
operation.
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As used herein, the term "subsurface" refers to geologic strata occurring
below the earth's
surface.
The term "low-friction coating," or "low coefficient of friction coating,"
refers to a
coating for which the coefficient of friction is less than 0.15.
As used herein, the term "wellbore" refers to a hole in the subsurface made by
drilling or
insertion of a conduit into the subsurface. A wellbore may have a
substantially circular cross
section, or other cross-sectional shapes. The term "well," when referring to
an opening in the
formation, may be used interchangeably with the term "wellbore." Note that
this is in contrast to
the terms "bore" or "cylinder bore" which may be used herein, and which refers
to a bore in a
tool.
Description of Selected Specific Embodiments
Figure 1A is a perspective view of a centralizer 100 as may be used in the
present
invention, in one embodiment. The centralizer 100 may be used for centering a
tubular body
such as a joint of casing, a liner, a joint of drill pipe, a production
tubing, an injection tubing, or a
sand screen in a wellbore. The centralizer 100 has an outer surface 110 and an
inner surface 115.
Figure 1B is a side view of the centralizer 100.
Figure 2A is a perspective view of a centralizer 200 as may be used in the
present
invention, in an alternate embodiment. The centralizer 200 may again be used
for centering a
tubular body such as a joint of casing or a liner string in a wellbore. The
centralizer 200 has an
outer surface 210 and an inner surface 215. Figure 2B is a side view of the
centralizer 200.
The centralizers 100, 200 generally have the same dimensions. Each centralizer
100, 200
includes a plurality of blades 120, 220 spaced around the outer surface 110,
210. In the
arrangement of Figures 1A and 1B, the blades 120 are substantially vertical;
in the arrangement
of Figures 2A and 2B, the blades 220 are angled. In each case, the
centralizers 100, 200 are
fabricated substantially from a steel material as a solid body. Further, the
blades 120, 220 define
at least two ridges along the respective outer surfaces 110, 210 spaced equi-
distantly around the
centralizer 100, 200.
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Figure 3 is a perspective view of a casing centralizer 300 as may be used in
the present
invention, in an alternate embodiment. Upon information and belief, the
illustrative centralizer
300 was designed by Top-Co Cementing Products, Inc. of Weatherford, Texas. The
casing
centralizer 300 has an outer surface 310 and an inner surface 315. Blades 320
reside around the
outer surface 310 in spaced-apart relation.
Figure 4 is a perspective view of a casing centralizer 400 as may be used in
the present
invention, in still another embodiment. The illustrative centralizer 400 was
also designed by
Top-Co Cementing Products, Inc. of Weatherford, Texas. The casing centralizer
400 has an
outer surface 410 and an inner surface 415. Blades 420 reside around the outer
surface 410 in
spaced-apart relation.
Figure 5 is a side view of a centralizer 500 of the present invention, in
another
embodiment. The centralizer 500 has a pair of spaced-apart collars 510. The
collars 510 are
designed to circumferentially receive the tubular body. In the view of Figure
5, a tubular body
is shown at 505, and is intended to represent a casing joint. Ideally, the
collars 510 fit loosely
around the tubular body 505, allowing the collars 510 to slide over the outer
diameter of the
tubular body 505. Preferably, the collars 510 are identical.
The centralizer 500 also has a plurality of leaf springs 520. The leaf springs
520 are equi-
distantly spaced, and are welded to the pair of collars 510 at opposing ends.
The leaf springs 520
have capability to "comply" with the diameter of a wellbore by bowing in and
out as the
centralizer 500 moves down hole.
The leaf springs 520 may be attached to the collars 510 in any manner.
Connection may
be, for example, by welding or by interlocking components.
The collars 510 and the leaf springs 520 may be fabricated from steel, from a
plastic
material, or from a ceramic material. Any such material is suitable so long as
the springs 520
have an element of elasticity to them, allowing them to bow in and out it as
the centralizer 500
moves through a wellbore. The centralizer 500 may be used for centering a
tubular body such as
a joint of casing, a liner, a production tubing, an injection tubing, or a
sand screen in a wellbore.
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Each collar 510 is made up of hinged connected accurate sections, in this case
two,
adapted to be wrapped around the casing 505 and then suitably latched to one
another by hinge
pins 518, all as well-known in the art.
It is observed, that during the drilling of a borehole through underground
formations, or
during the running of a casing string into a wellbore, the string of pipe
undergoes considerable
rotational and sliding contact with the rock formations. Further, considerable
relative rotation
and translation occurs between the pipe string and the surrounding
centralizers. Accordingly, in
each of the illustrative centralizers 100, 200, 300, 400, 500, a low friction
coating is applied at
least to the inner surfaces 115, 215, 315, 415, 515.
In traditional drilling and completion operations, a lubricating drilling mud
is pumped
into the wellbore. The drilling mud may be either a water-based or an oil-
based mud. Diesel and
other mineral oils are also often used as lubricants. Minerals such as
bentonite are known to help
reduce friction between the pipe strings downhole and an open borehole.
Materials such as
Teflon have also been used to reduce friction, however these lack durability
and strength. Other
additives include vegetable oils, asphalt, graphite, detergents and walnut
hulls, but each has its
own limitations.
Yet another method for reducing the friction between a pipe string, typically
a drill string
and the borehole is to use a hard facing material (also referred to in the
industry as
"hardbanding"). U.S. Patent No. 4,665,996 discloses the use of hardbanding the
bearing surface
of a drill pipe with an alloy having the composition of: 50-65% cobalt, 25-35%
molybdenum, 1-
18% chromium, 2-10% silicon and less than 0.1% carbon for reducing the
friction between the
drill string and the rock matrix. As a result, the torque needed for rotary
drilling operations is
decreased. Another form of hardbanding is WC-cobalt cermets applied to a drill
stem assembly.
Other hardbanding materials include TiC, Cr-carbide, Nb-carbide and other
mixed carbide,
carbonitride, boride and nitride systems. Hardbanding may be applied to
portions of a drill string
or a directional drilling assembly using weld overlay or thermal spray
methods.
To reduce the coefficient of friction between the joint of casing (such as
casing 505) and
the surrounding centralizer, and in lieu of the above known methods, it is
proposed herein to coat
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the inner surface with a low-coefficient of friction material. The low-
friction material is
preferably a Molykote anti-friction coating available from Dow Corning Corp.
of Midland,
Michigan, having Molybdenum disulfide (M0S2). Alternatively, it is proposed
herein to create a
low-coefficient of friction layer using a terrific nitro-carburizing process
that produces a
polytetrafluoroethylene (PTFE) coating on all surfaces.
Ferritic nitro-carburizing ("FNC"), also known as soft nitriding, is applied
to carbon
steels, tool steels, alloy steels and stainless steels to provide anti-galling
wear resistance. The
procedure is used in the auto industry to improve the fatigue life of car
parts. The procedure is
also used to enhance the wear characteristics of forging and stamping dies,
fixtures, gears and
molds.
FNC is a form of heat treating. Different heat treating companies apply their
own
proprietary gas compositions, gas flow rates, and furnace temperatures to
produce the right nitro-
carburizing environment. Some companies have developed unique processes for
nitriding,
including so-called Salt Bath FNC, Fluidized-Bed FNC and Plasma (or Ion) FNC.
However, it
has been observed, particularly with Gaseous FNC where gas compositions are
injected into a
chamber at high temperatures, that the resulting coating creates an outer
layer having very low
relative friction.
Figures 6 and 7 present flow charts showing steps for methods of fabricating a
centralizer, in alternate embodiments.
Referring first to Figure 6, a first method 600 for fabricating a centralizer
is provided.
The method 600 first includes providing a centralizer. This is shown in Box
610. The
centralizer comprises an elongated body having a bore there through. The bore
is dimensioned
to receive a tubular body such as a joint of casing. The elongated body has an
inner surface and
an outer surface. Preferably, the body is a substantially solid metallic
material, though it may
optionally include small perforations. The outer surface of the body may have
two or more
blades forming channels for carrying or directing a fluid.
The method 600 also includes depositing a low-coefficient coating onto the
inner surface
of the body. This is seen in Box 620. The coating is designed to provide a
reduced coefficient of
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friction on the inner surface. In one aspect, the coating has a coefficient of
friction that is about
0.1.
The low-friction material is preferably the Molykote coating available from
Dow
Corning Corp. of Midland, Michigan. In one aspect, the Molykote 3402-C anti-
friction coating
is used. This coating is a blend of solid lubricants, corrosion inhibitors,
and an organic binder
dispersed in a solvent. This coating can be applied directly to a steel
surface and will generally
cure within 2 hours at room temperature, and in less than 10 minutes at higher
temperatures.
The Molykote 3402-C anti-friction coating forms a slippery film that covers
the surface
of the centralizer to reduce friction against the casing joint. Such an anti-
friction coating is
beneficial as it allows for a dry, clean lubricant between the steel pipe and
the surrounding
centralizer while being run down hole, reducing the drag coefficient.
The anti-friction coating may be brushed, dipped, heat sprayed, or cold wet
sprayed onto
the subject surface of the centralizer. Preferably, the coating is sprayed
onto the surface using a
centrifugal sprayer. The centralizer may be cooled while the coating is
allowed to cure.
It is noted that additional Molykote formulations may be used as the anti-
friction
coating. One such variety is the Molykote 7400 anti-friction coating. This is
a water dilutable
coating that can be applied using a centrifugal sprayer, and then kiln dried
at about 20 C in
about fifteen minutes.
Preferably, the surface is pre-treated using phosphatization or
sandblasting to increase adhesion. After application, a maintenance free
coating is left.
Other low-friction coating materials include polytetrafluoroethylene (PTFE),
or TeflonTm.
Alternatively, low-friction coating materials include perfluoroalkoxy polymer
resin (PFA),
fluorinated ethylene propylene copolymer (FEP), ethylene
chlorotrifluoroethylene (ECTFE), and
the copolymer of ethylene and tetrafluoroethylene (ETFE).
Other suitable low-friction materials include polyetheretherketone, carbon
reinforced
polyetheretherketone, polyphthalamide, polyvinylidene fluoride (PVDF),
polyphenylene
sulphide, polyetherimide, polyethylene (PE) and polysulphone.
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Certain of the low-friction coating materials listed above are available in
products under
the brand names:
Molykote available from Dow Corning Corp. of Midland, Michigan (as noted);
WearlonTM available from Plastic Maritime Corp. of Wilton, New York;
Halar available from Solvay Solexis, Inc. of Thorofare, New Jersey;
Kynar available from Arkema, Inc. of King of Prussia, Pennsylvania;
Vydax and SilverstoneTM available from E.I. Du Pont De Nemours and Co. of
Wilmington, Delaware;
Dykor available from Whitford Corp. of West Chester, Pennsylvania;
Emralon available from Henkel Corp. of Rocky Hill, Connecticut;
ElectrofilmTM available from Orion Industries of Chicago, Illinois; and
Everlube available from Metal Improvement Company, LLC of East Paramus,
New Jersey.
In another aspect, a low-coefficient of friction coating is used that contains
graphite or
graphite powder. Graphite is an allotrope of carbon. Alternatively, the
coating may include
Molybdenum disulfide (MoS2), which is a black crystalline sulfide of
molybdenum.
Alternatively still, the coating may include hexagonal Boron Nitride (hBN),
also known as
"White Graphite." This dry material in powder form is known to reduce friction
between solid
bodies. Combinations thereof may be used, applied using a brushing, dipping or
spraying
process.
The method 600 also comprises allowing the low-coefficient coating to cure on
the inner
surface. This is indicated at Box 630. Curing may be done by heating or by air
drying. The
low-coefficient coating may meet ASTM-D2714 or ASTM-D2625 standards to form a
slippery
film, optimizing metal-to-metal friction control.
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Optionally, the method 600 further includes depositing a low-coefficient of
friction
coating onto the outer surface. This is seen in Box 640. Here, the coating is
designed to provide
a reduced coefficient of friction on the outer surface. The coating may be any
of the low-friction
coatings listed above.
The method 600 then comprises allowing the low-coefficient coating to cure on
the outer
surface. This is provided at Box 650.
It is observed that the above materials may be applied to the inner surface,
the outer
surface, or both, of a centralizer by first cleaning and degreasing the
surface. The cleaner the
surface, the better the highly lubricious material will adhere. The subject
surface may then be
lightly sanded or, alternatively, sand blasted, such as by using a 5-micron
Alumina (Aluminum
Oxide) powder. The centralizer is then manually cleaned using a soft cloth.
Then, the
centralizer is again sand blasted, but this time with a selected dry
lubricating powder, or
combinations thereof, therein. Blasting may be done, for instance, at 120 psi
using clean and
cold pneumatic air. The centralizer is sprayed until the outer surface begins
to change color, e.g.,
silver-gray. The surface is then again lightly buffed.
In another aspect, the surfaces of the centralizer are coated with an ultra-
low friction
diamond-like-carbon (DLC) coating. The DLC coating may be chosen from
tetrahedral
amorphous carbon (ta-C), tetrahedral amorphous hydrogenated carbon (ta-C:H),
diamond-like
hydrogenated carbon (DLCH), polymer-like hydrogenated carbon (PLCH), graphite-
like
hydrogenated carbon (GLCH), silicon containing diamond-like carbon (Si-DLC),
metal
containing diamond-like carbon (Me-DLC), oxygen containing diamond-like carbon
(0-DLC),
nitrogen containing diamond-like carbon (N-DLC), boron containing diamond-like
carbon (B-
DLC), fluorinated diamond-like carbon (F-DLC), or combinations thereof.
The DLC coatings may be deposited by physical vapor deposition. The physical
vapor
deposition coating methods include RF-DC plasma reactive magnetron sputtering,
ion beam
assisted deposition, cathodic arc deposition and pulsed laser deposition
(PLD). In sputter
deposition, a glow plasma discharge (usually localized around a source
material by a magnet)
bombards the material, sputtering some material away as a vapor for subsequent
deposition. In
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cathodic arc deposition, a high-powered electric arc is discharged at a source
material to blast
away portions into a highly ionized vapor, that is then deposited onto a work
piece. In ion (or
electron) beam deposition, the material to be deposited is heated to a high
vapor pressure by
electron bombardment in a high-vacuum environment, and then transported by
diffusion to be
deposited by condensation on the (cooler) work piece. In pulsed laser
deposition, a high-power
laser ablates material from a target (source material) into a vapor. The
vaporized material is then
transported to the work piece and deposited.
Chemical vapor deposition may also be used as a coating technique. Chemical
vapor
deposition coating methods include ion beam assisted CVD deposition, plasma
enhanced
deposition using a glow discharge from hydrocarbon gas, using a radio
frequency glow discharge
from a hydrocarbon gas, plasma immersed ion processing and microwave
discharge. Plasma
enhanced chemical vapor deposition (PECVD) is one advantageous method for
depositing DLC
coatings on large areas at high deposition rates. Plasma-based CVD coating
process is a non-
line-of-sight technique, i.e. the plasma covers the part to be coated and the
entire exposed surface
of the part is coated with uniform thickness.
In an alternate embodiment of the method 600, the step 620 is modified so that
the low-
coefficient coating is sand blasted onto the surface rather than deposited. In
this instance, the
step 630 of allowing the coating to cure is replaced with a step of buffing
the surface.
Figure 7 provides a second method of manufacturing a casing centralizer.
Figure 7 is a
flow chart showing steps for a method 700 of manufacturing a centralizer, in
an alternate
embodiment. The centralizer is fabricated from a metal material, such as
steel. The method 700
employs a vapor deposition process.
The method 700 first involves forming a centralizer through a milling (or
cutting)
process. This is provided at Box 710. As an alternative, a molding process may
be employed.
The centralizer is formed to have a bore defining inner and outer surfaces.
The inner surface is
dimensioned to lightly engage the outer surface of a wellbore pipe.
Preferably, the outer surface
comprises blades equi-distantly spaced about an outer diameter of the
centralizer.
CA 02858624 2014-08-07
The method 700 also includes placing the centralizer into a vapor deposition
chamber.
This is shown at Box 720.
The method 700 further includes a heating step. This is indicated at Box 720.
Heating
may mean heating the chamber to a temperature in excess of 750 F. More
preferably, heating
means heating the chamber to about 950 F to 1,150 F. The processing of
heating the chamber
also heats the metal material making up the centralizer.
Alternatively, the heating step of Box 720 may mean heating the centralizer
directly.
This may be by using a plasma torch. The plasma torch enables heating of the
downhole device
to a very high temperature, even in excess of 2,500 F.
The method 700 may optionally include applying a vacuum within the deposition
chamber. This is seen at Box 740. Applying a vacuum serves to lower the
pressure in the
chamber, thereby assisting the vapor deposition process. In one aspect, the
pressure is lowered
to between about one and ten tons.
As a next step in the method 700, a vapor is injected into the deposition
chamber. This is
provided at Box 750. It is understood that vapor may be a gas that is below
its critical
temperature. Preferably, the vapor is injected through one or more atomizing
nozzles. A
gaseous mixture comprising nitrogen and carbon may be injected through the one
or more
nozzles.
In one aspect, each nozzle injects a different inert gas. In another aspect, a
pre-mixed
composition of gases is injected through each of the nozzles. Gases may
include ammonia,
carbon, hydrogen and other gases. In any event, the gas atoms locate onto the
centralizer
structure. Further, during the heating step 730, the metal material making up
the centralizer
expands, allowing the gaseous mixture to penetrate into the structure of the
metal material as
nano-particles. It is preferred that the heating and vapor deposition process
be conducted over a
period of about one hour. Thus, the method 700 also includes continuing to
heat the deposition
chamber after vapor deposition.
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After heating, the deposition chamber and the centralizer located therein are
allowed to
cool. This is provided at Box 760. As the centralizer cools within the
deposition chamber, the
inert nano-particles become trapped or embedded into the metal material,
primarily at the surface
of the centralizer. In this way, a non-friction coating is formed along both
inner and outer
surfaces of the centralizer. (It is understood that for purposes of this
disclosure, the term
"coating" includes any layer proximate a surface of the centralizer.)
The method 700 may be a Gaseous FNC process. The gases injected through the
nozzles
may include carbon, nitrogen, ammonia and an endothermic gas. The centralizer
is preferably
cleaned using a vapor degreasing process, and then nitrocarburized at a
chamber temperature of
between about 950 F to 1,150 F. The FNC process may be the method disclosed
in U.S. Patent
Pub!. No. 2011/0151238, entitled "Low-Friction Coating System and Method." The
application
teaches a method that includes the steps of:
ferritic nitro-carburizing a metal substrate to form a surface of the
metal substrate including a compound zone and a diffusion zone disposed
subjacent to the compound zone;
after ferritic nitro-carburizing, oxidizing the compound zone to
form a porous portion defining a plurality of pores;
after oxidizing, coating the porous portion with
polytetrafluoroethylene; and
after coating, curing the polytetrafluoroethylene to thereby form
the low-friction coating.
It is preferred that a first coating be applied using a nitriding process. The
nitriding
process may be an FNC process whereby nano-particles comprising nitrogen and
carbon
are diffused into the metal surfaces of the centralizer. Ammonia may be used
as a nitrogen
source. The surfaces are allowed to cool. Thereafter, a second coating is
applied that
contains graphite or molybdenum disulfide. Alternatively, the second coating
may be a
diamond-like-carbon (DLC) coating or PTFE.
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A method of setting casing in a wellbore is also provided herein. Figure 8 is
a flow chart
showing steps for a method 800 of setting a casing string in a wellbore, in
one embodiment.
The method 800 first comprises running joints of casing into a wellbore. This
is shown
in Box 810. The joints of casing are threadedly connected end-to-end as they
are lowered into
the wellbore.
The method 800 also includes attaching one or more centralizers to selected
joints of
casing as the joints of casing are lowered into the wellbore. This is provided
in Box 820. Each
of the one or more centralizers comprises an elongated body having a bore
there through. The
bore is dimensioned to slidingly receive a joint of casing. The elongated body
has an inner
surface and an outer surface.
In a preferred aspect, each of the centralizers is a substantially solid and
metallic body
having blades equi-distantly spaced around the outer surface. Each of the
centralizers has a
coating deposited on at least the inner surface, wherein the coating is
designed to provide a
reduced coefficient of friction. The coating may be any of the coatings
described or listed above.
Additional technical information concerning low-friction coatings in the
context of downhole
operations is provided in U.S. Patent No. 8,220,563 entitled "Ultra-Low
Friction Coatings for
Drill Stem Assemblies".
In one aspect, the coefficient of friction is lower on the inner surface after
curing or after
buffing than on the outer surface.
The method 800 further includes injecting a cement slurry into an annular
space formed
between the joints of casing and the surrounding wellbore. This is indicated
at Box 830.
Injecting the slurry generally means pumping the cement slurry down a bore of
the casing string,
down to a cement shoe or bottom of the casing string, and back up the annular
space.
The method 800 also includes allowing the cement slurry to set. This is
provided at Box
840. In this way, the casing string with the centralizers is set in the
wellbore.
It is noted that the centralizers presented above in Figures 1 through 5 are
merely
illustrative. Any centralizer design may be used with the low-friction coating
to reduce the drag
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and torque coefficients of friction between the casing and the centralizers.
Preferably, the
coefficient of friction is less than 0.15. More preferably, the coefficient of
friction is less than
about 0.10.
As can be seen, an improved centralizer is offered that reduces the
coefficient of friction
between a joint of casing in a wellbore, and a surrounding centralizer. The
reduced coefficient of
friction enables the centralizer to move along an outer surface of casing
joints without damaging
the casing or creating stress joints. Dimensions of the centralizer may be
adjusted during
manufacturing for use on hardbanded drill pipe. The ferfitic nitro-carburizing
process is
preferred, followed by a second coating comprising graphite, molybdenum
disulfide, PTFE, or a
diamond-like-carbon on all surfaces. The ferritic nitro-carburizing process
beneficially increases
the durability of the centralizer for its wellbore operations.
The scope of the claims that follow is not limited by the embodiments set
forth in the
description. The claims should be given the broadest purposive construction
consistent with the
description and figures as a whole.
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